THE TRANSPORTATION OF GASES
169
effective pressure and the initial pressure for various pressure ratios, pa/pi, with adiabatic compression and complete intercooling.
^ The ideal method of compressing air when it is to be stored or allowed to cool before being used is the isothermal, and in the earlier types of compressors this was attempted by so-called wet compressors, by means of which it was possible to secure compressions approximating plVl™ == p,t,,i-». The mechanical difficulties involved and the necessary low speeds with consequent small capacity have led to the use of modern "dry compressors," which in small sizes have cylinders with cast-iron ribs for radiating heat, and in large sizes have water jackets surrounding the cylinder. The cooling thus secured is sufficient to keep temperatures from being excessive, but as a rule the compression curves are above pv1-3 = constant. Dry compression has the advantage of higher speeds and larger capacities.
Efficiencies.-— The effect of clearance upon capacity is usually expressed in terms of volumetric efficiency.
The apparent volumetric efficiency is the apparent volume of free air drawn in (as shown by the indicator card) divided by the volume of the piston displacement. Inis is the term that is commonly used in speaking of volumetric efficiency, and m Fig. 29 it is GK/L. If the clearance expansion line follows the equation Plv? =
p*>f, where the clearance C = v^/L, then v,/L = (Pl/p^C. The volumetric efficiency may also be written:
16
8 \l
'Values of PS..
FIG. 27.—Mean effective pressures in air compression.
Example.— It Pl is 94.7 Ib. per square inch absolute and C is 2 per cent., the volumetric efficiency will be 1 - 0.02 [ (vH) *'4 - l] = 0.9444.
04 & IZ 16 M JU
FIG. 28.—Air compressor card.
FIG. 29.
The loss in capacity for stage compression will be represented by the following formulas in which Pl is the initial and p2 the final pressure:
i
Two-stage: 1 -c[(|2)ix1-4 -ll; Three-stage:! -C\(»^~^ _ {\
- L V)l' J
T?- on Vl
figure 29 represents graphically the part within the brackets of the above equations. Knowing the actual clearance and the pressure range, the effect